1. Field of the Invention
The present invention relates to a driving device, and in detail, to a driving device and a driving method utilizing an electromechanical conversion element such as a piezoelectric element. A driving device of the present invention is suitable for a lens driving mechanism in a camera and a driving mechanism of a precision stage, for example.
2. Description of the Related Art
Recently, a driving device using a piezoelectric element is proposed for driving a member constituting precision equipment such as a camera and an information recording/reproducing device. Such a device is so configured that by utilizing a phenomenon of a piezoelectric element causing a distortion in a lengthwise direction when a voltage is applied, an impulse voltage is applied to the piezoelectric element so as to give an impact force to an inertial body movably engaging with the piezoelectric element, whereby a driven body is driven by utilizing the motion of the inertial body caused by the continuous impact force.
Japanese Patent Application Laid-open No. 11-98865 shows a driving device utilizing an electromechanical conversion element such as a piezoelectric element in which the length changes (extends and contracts) when a voltage is applied, in an exploded perspective view of FIG. 1(a) and an assembly perspective view of FIG. 1(b), for example.
This driving device is capable of moving a moving body 10 relatively to a fixed member 1, and can be used as a lens driving device of a camera, for example. That is, when the moving body 10 is coupled to a lens frame, it is possible to move a lens together with the moving body 10.
A piezoelectric element 4 is composed of a number of laminated piezoelectric plates. One end 4a thereof in an extension/contraction direction is fixed to the fixed member 1, and the other end 4b is fixed to a first end 5a of a rod 5. The rod 5 is supported slidably by supporting parts 2 and 3 integrally formed with the fixed member 1.
The moving body 10 is frictionally coupled around the rod 5 in such a manner that the rod 5 is interposed between a body 11 and a cap 12, and the body 11 and the cap 12 are given an energizing force by a press spring 13 in an interposing direction.
The piezoelectric element 4 is connected with a voltage control circuit (a driving pulse generating means) not shown. When a predetermined driving voltage of a sawtooth waveform is applied to the piezoelectric element 4, the piezoelectric element 4 vibrates with a sawtooth displacement of the almost same form (FIG. 2). Along with it, the rod 5 also vibrates in a lengthwise direction thereof with a sawtooth displacement. That is, the graph of FIG. 2 shows the vibration displacement of the piezoelectric element 4 as well as the vibration displacement of the rod 5.
Specifically, at a gently rising slope part 101 in a period A of a first waveform 100, the piezoelectric element 4 extends relatively slowly, and the rod 5 moves slowly in the arrow I direction in FIG. 1(b). Next, in a period B, the piezoelectric element 4 contracts quickly so as to return to be in the initial length (a waveform portion shown by the rising slope part 102), and the rod 5 rapidly moves to the arrow II direction. Like movements are repeated, and consequently, the rod 5 vibrates while repeating slow movements in the I direction and rapid movements in the II direction. In this way, the rod 5 vibrates while forming slow and rapid sawtooth vibration waveforms as shown in FIG. 2.
Here, as shown in FIG. 3, the elastic force of the press spring 13 of the moving body 10 (frictionally coupling force of the moving body 10 to the rod 5) is regulated such that the moving body 10 moves together with the rod 5 when the rod 5 moves slowly, and the moving body 10 remains at the position due to the inert (or moves a smaller amount than the rod 5) when the rod 5 moves rapidly. Therefore, during the vibration of the rod 5, the moving body 10 moves in the I direction relatively to the fixed member 1.
Note that when moving the moving body 10 in the arrow II direction in FIG. 1(b), the vibration waveforms of the piezoelectric element 4 and the rod 5 should be opposite to those shown in FIG. 2, that is, the waveform should consist of a sharp rising part and a gentle falling part. The moving principle of the moving body 10 is same as that described above.
As described above, although it is required to apply a driving voltage of a sawtooth waveform to the piezoelectric element, Japanese Patent Application Laid-open No. 11-98865 discloses two methods of generating such a driving voltage.
A first method is one using a waveform generator and an amplifier, as shown in FIGS. 4(a), 4(b) and 4(c). As shown in FIG. 4(a), a sawtooth waveform of 8 bits, 0–5 V is generated by DA conversion of the waveform generator, which is amplified to 1–10 V by using a power amplifier. In this way, a sawtooth driving waveform of 0–10 V is obtained. FIG. 4(b) shows a driving voltage waveform for moving the moving body 10 in the I direction in FIG. 3, and FIG. 4(c) shows a driving voltage waveform for driving it in the opposite direction.
A second method is one using a constant current circuit and a switching circuit, as shown in FIGS. 5(a) and 5(b). In a digital circuit shown in FIG. 5(a), A and D constitute constant current circuits, and B and C constitute switching circuits. To this digital circuit, a signal shown in FIG. 5(b) is given to terminals a to d. Thereby, the constant current circuits A and D and the switching circuits B and C are activated in turn, so that a sawtooth driving waveform of 0–10 V is obtained. As described above, in order to obtain a sawtooth driving voltage waveform, it is required conventionally to use a waveform generator and an amplifier, or to use a constant current circuit and a switching means. Therefore, the configuration is complicated, causing a cost increase.